A new view of mechanotransduction and strain amplification in cells with microvilli and cell processes

In this paper we shall describe new mechanical models for the deformation o f the actin filament bundles in kidney microvilli and osteocytic cell proce sses to see whether these cellular extensions, like the stereocilia on hair cells in the inner ear, can function as mechanotransducers when subject to physiological flow. In the case of kidney microvilli we show that the hydr odynamic drag forces at the microvilli tip are <0.01 pN, but there is a 38- fold force amplification on the actin filaments at the base of the microvil li due to the resisting moment in its terminal web. This leads to forces th at are more than sufficient to deform the terminal web complex of the micro villus where ezrin has been shown to couple the actin cytoskeleton to the N a+/H+ exchanger. In the case of bone cell processes we show that the actin filament bundles have an effective Young's modulus that is 200 times > the measured modulus for the actin gel in the cell body. It is, therefore, unli kely that bone cell processes respond in vivo to fluid shear stress, as pro posed in [59]. However, we show that the fluid drag forces on the pericellu lar matrix which tethers the cell processes to the canalicular wall can pro duce a 20-100 fold amplification of bone tissue strains in the actin filame nt bundle of the cell process.